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Abstract

This paper reviews over 30 years of work on photonic analog-to-digital
converters. The review is limited to systems in which the input is a
radio-frequency (RF) signal in the electronic domain and the output is a digital
version of that signal also in the electronic domain, and thus the review
excludes photonic systems directed towards digitizing images or optical
communication signals. The state of the art in electronic ADCs, basic properties
of ADCs and properties of analog optical links, which are found in many photonic
ADCs, are reviewed as background information for understanding photonic ADCs.
Then four classes of photonic ADCs are reviewed: 1) photonic assisted ADC in
which a photonic device is added to an electronic ADC to improve performance, 2)
photonic sampling and electronic quantizing ADC, 3) electronic sampling and
photonic quantizing ADC, and 4) photonic sampling and quantizing ADC. It is
noted, however, that all 4 classes of “photonic ADC”
require some electronic sampling and quantization. After reviewing all known
photonic ADCs in the four classes, the review concludes with a discussion of the
potential for photonic ADCs in the future.

Input voltage as a function of time (green) and the sampled and quantized
voltage as a function of time (red). The upper row shows digitization of a
noiseless signal with N = 3, 4, and 5. The lower row shows digitization of
the same signal plus noise with N = 3, 4, and 5. In the upper row one can
clearly see the additional benefit of higher numbers of bits N (for N = 5
look at t = 0-1, 5-7 and 9-10) and of course, the ENOB equals the number of
bits N through the definition in eqs. (2) and (3). In the lower row, one sees a benefit in increasing
the bits from 3 to 4, but no apparent improvement is obtained by increasing
N from 4 to 5 because the ENOB is limited to about 4 by the noise on the
signal and not by the quantization noise.

Optically quantized photonic ADC based on tuning the wavelength of an optical
source, reflecting that source from a diffraction grating and focusing the
output through a diffractive optical element to an array of detectors.
(Adapted from [134])